Final Exam

FTP Server Assignment 10

Application Layer 10

Transport 10

Network 10

Link-Layer 10

Security 10

TotalTotal 6060

Coverage and distribution of marks

NoteNote

• Wireless Networking is not going to be included in the Final Exam.

Types of QuestionsTypes of Questions

• Define terms, identify technologies, explain purpose

• Explain concepts and protocols– Using diagrams (label them properly)– Appropriate variables (or parameters)

• Solve problems (e.g. apply DV Algorithm, CRC)

MAC Protocols: a taxonomyMAC Protocols: a taxonomy

Three broad classes:Three broad classes:• Channel Partitioning Protocols

– divide channel into smaller pieces (time slots, frequency bands, multiple access codes)

– allocate piece to node for exclusive use

• Random Access Protocols

– allow collisions– recover from collisions

• Taking turns Protocols– tightly coordinate shared access to avoid collisions

Animation

Channel Partitioning (CDMA)Channel Partitioning (CDMA)

CDMA (Code Division Multiple Access) • Sender: sends encoded data bits simultaneously• Receiver: assigned a unique code (i.e. code set partitioning)• Has been used by military; now used mostly in wireless

broadcast channels (cellular, satellite,etc)• all users share same frequency, but each user has own

''chipping'' sequence (i.e., code) to encode data• encoded signal = (original data) X (chipping sequence)• decoding: inner-product of encoded signal and chipping

sequence• Advantage: allows multiple users to coexist and transmit

simultaneously with minimal interference (if codes are orthogonal)

CDMA Encode/DecodeCDMA Encode/Decode

MM mini slots are assigned to each data

bit

CDMA: two-sender interferenceCDMA: two-sender interference

Assumption: interfering bit signals are additive

Random Access protocolsRandom Access protocols

• When node has packet to send– transmit at full rate (R) of channel– no a priori coordination among nodes

• two or more transmitting nodes -> collision!!,• random access MAC protocol specifies:

– how to detect collisions– how to recover from collisions (e.g., via delayed

retransmissions)

• Examples of random access MAC protocols:– slotted ALOHA– ALOHA– CSMA and CSMA/CD

CSMA: CSMA: (Carrier Sense Multiple Access)(Carrier Sense Multiple Access)

CSMA: listen before transmit:• If channel sensed idle: transmit entire pkt• If channel sensed busy, defer transmission

– Persistent CSMA: retry immediately with probability pp when channel becomes idle (may cause instability)

– Non-persistent CSMA: retry after random time interval

• human analogy: don't interrupt others!

RULES: •Listen before speaking (carrier sensing)• If someone else begins talking at the same time, stop talking (collision detection)

Animation

CSMA collisions

collisions can occur:

propagation delay means two nodes may not yethear each other's transmissioncollision:

entire packet transmission time wasted

spatial layout of nodes along ethernet

note:

role of distance and propagation delay in determining collision prob.

Channel propagation delay

4 nodes attached to a linear broadcast bus; Horizontal axis shows position of

each node in space

D senses the channel is idle at t1

CSMA/CD (Collision Detection)CSMA/CD (Collision Detection)

CSMA/CD: carrier sensing, deferral as in CSMA– collisions detected within short time– colliding transmissions aborted, reducing channel

wastage – persistent or non-persistent retransmission

• collision detection: – easy in wired LANs: measure signal strengths,

compare transmitted, received signals– difficult in wireless LANs: receiver shut off while

transmitting

• human analogy: the polite conversation

CSMA/CD collision detectionCSMA/CD collision detection

4 nodes attached to a linear broadcast

bus

From Network to Link-Layer

Datagram to Frame

Recall earlier routing discussion

223.1.1.1

223.1.1.2

223.1.1.3

223.1.1.4 223.1.2.9

223.1.2.2

223.1.2.1

223.1.3.2223.1.3.1

223.1.3.27

A

BE

Starting at A, given IP datagram addressed to B:

look up net. address of B, find B on same net. as A

link layer send datagram to B inside link-layer frame

B’s MACaddr

A’s MACaddr

A’s IPaddr

B’s IPaddr

IP payload

datagramframe

frame source,dest address

datagram source,dest address

Additional addresses!

ARP: Address Resolution Protocol

• Each IP node (Host, Router) on LAN has ARP module, table

• ARP Table: IP/MAC address mappings for some LAN nodes

< IP address; MAC address; TTL>

<………….. >– TTL (Time To Live): time after

which address mapping will be forgotten (typically 20 min)

Question: how to determineMAC address of Bgiven B's IP address?

ARP for Ethernet (RFC 826), Intro to ARP,TCP/IP Tutorial (RFC 1180)

Resides in RAM

ARP: Address Resolution ProtocolHow is ARP Table initialized?

• It’s plug-and-play!• Not configured by Network Administrator• Tables are automatically updated when one node is disconnected from subnet

ARP protocol: A Scenario• A knows B's IP address, wants to learn physical address of B • A broadcasts ARP query pkt, containing B's IP address

– all machines on LAN receive ARP query

• B receives ARP packet, replies to A with its (B's) physical layer address

• A caches (saves) IP-to-physical address pairs until information becomes old (times out) – soft state: information that times out (goes away) unless refreshed

Passes ARP packet within the frame up to its parent node

Only one node (node with IP address=DEST IP in the ARP packet) sends back to the

querying node a response ARP packet with the desired mapping

In turn, the querying node can then update its ARP Table and send its IP datagram

Routing to another LAN

A

RB

Network 1: Network address: 111.111.111/24

Network 2: Network address: 222.222.222/24

Two Types of Nodes: • Hosts• Routers

Router• has an IP address for each of its interfaces• has an ARP module for each of its interfaces• has an adapter for each of its interfaces

• each adapter has its own MAC Address2 interfaces, 2 IP addresses, 2 ARP modules, 2 adapters (2

MAC addresses)

Routing to another LANrouting from A to B via R

In routing table at source Host, find router 111.111.111.110• In ARP table at source, find MAC address E6-E9-00-17-BB-

4B, etc

A

RB

Network 1: Network address: 111.111.111/24

Network 2: Network address: 222.222.222/24

• A creates IP packet with source A, destination B

A

RB

How a HOST on Subnet 1 sends a datagram to a HOST on Subnet 2?

Passes datagram to adapter Host A must indicate to its adapter an appropriate MAC

address (DEST)

Note: Before sending the frame, MAC address must be acquired (should match one of the nodes in subnet; otherwise, will be lost). Then, frame is sent to Subnet.

• A uses ARP to get R's physical layer address for 111.111.111.110 (adapter side)

A

RB

Note: Router interface• handles routing; uses Forwarding Table

Adapter • uses ARP to find MAC address

• (adapter) A creates Ethernet frame with R's physical address as dest, Ethernet frame contains A-to-B IP datagram

• (adapter) A's data link layer sends Ethernet frame • (adapter) R's data link layer receives Ethernet frame

– Passes frame to Network Layer• (network layer) R removes IP datagram from Ethernet frame,

sees its destined to B– Using router R’s Forwarding table) – table tells R to forward

datagram via router interface 222.222.222.220• R’s interface passes datagram to its adaptor

A

RB

• (adapter) R uses ARP to get B's physical layer address • (adapter) R creates frame containing A-to-B IP datagram

sends to B

A

RB

CRC

Pseudo code, Manual Calculation of CRC

See sample computations sample

Ethernet: uses CSMA/CD

A: sense channel, if idle then {

transmit and monitor the channel; If another transmission is detected

then {

abort and send jam signal;

update # collisions;

delay as required by exponential backoff algorithm;

goto A

}

else {done with the frame; set collisions to zero}

}

else {wait until ongoing transmission is over and goto A}

Run without explicit coordination with other adapters on the Ethernet

Connectionless unreliable service to

Network Layer

No signal energy in channel for 96 bit

times

Ethernet’s CSMA/CD (more)Jam Signal: make sure all other transmitters are aware

of collision; 48 bits;

Exponential Backoff: • Goal: adapt retransmission attempts to estimated

current load– heavy load: random wait will be longer

• first collision: choose K randomly from {0, 2m-1};

delay is K x 512 bit transmission times• after second collision: choose K from {0,1,2,3}…• after ten or more collisions, choose K from {0,1,2,3,4,

…,1023}

=min(10, num of collisions)

For 10Mbps Ethernet, time to transmit 1 bit = 0.1 microseconds

Network Devices

Repeaters, Hubs, Bridges, Routers, Switches

Hubs• Physical Layer devices: essentially repeaters

operating at bit levels: repeat received bits on one interface to all other interfaces

• Hubs can be arranged in a hierarchy (or multi-tier design), with backbone hub at its top

Definition of Terms

Backbone Bridge

Bridges Spanning Tree• for increased reliability, it is desirable to have

redundant, alternate paths from source to dest• with multiple simultaneous paths, cycles result -

bridges may multiply and forward frame forever• solution: organize bridges in a spanning tree by

disabling subset of interfaces

Disabled

Bridge Learning: example

Suppose C sends a frame to D and D replies back with a frame to C

C sends the frame to the bridge, but the bridge has no info. about D, so it floods

both LANs bridge notes that C is on port 1 frame ignored on upper LAN frame received by D

Bridge Learning: example

D generates a reply to C, sends bridge sees frame from D bridge notes that D is on interface 2 bridge knows C on interface 1, so it selectively forwards the frame out via

interface 1

Routers vs. Bridges

Routers + and -+ arbitrary topologies can be supported, cycling is limited

by TTL counters (and good routing protocols)

+ provide firewall protection against broadcast storms

- require IP address configuration (not plug and play)

- require higher processing bandwidth

• bridges do well in small (few hundred hosts) while routers used in large networks (thousands of hosts)

Ethernet Switches (more)

Dedicated

Shared

Institutional Network using a combination of hubs, Ethernet switches, router

Interconnection DevicesCOMPARISON OF FEATURES

HUBS BRIDGES ROUTERSETHERNET SWITCHES

Traffic Isolation

No Yes Yes Yes

Plug-and-Play

Yes Yes No Yes

Optimal Routing

No NoYes No

Cut-through

Yes No No Yes

Review Pseudo codes as well

Let’s see some captured runs of the DV algorithm

Node Update

0 1 2 3 Min

0 999 999 999 999 0

1 999 1 999 999 1

2 999 999 3 999 3

3 999 999 999 7 7

Min

0 3

1 70

2 0

3 2

Rcv event, t=1.484 at 0; Source: 2, Dest: 0

Node 0

srtpkt2

0 1 2 3 Min

0 999 999 999 999 0

1 999 1 73 999 1

2 999 999 3 999 3

3 999 999 5 7 5

3 + 70 = 73

Node Update

Min

0 0

1 1

2 3

3 5

Apply Poisoned ReverseNode 0

srtpkt0

0 1 2 3 Min

0 999 999 999 999 0

1 999 1 999 999 1

2 999 999 3 999 3

3 999 999 5 7 5

73 turns 999

Send new mincost to neighbors via tolayer2()

DV AlgorithmThere are four routers connected via certain links (figure). Suppose that the following table

is used for the initialisation of C’s routing table:

Cvia

A B D

dest

A 1 ∞ ∞

B ∞ 3 ∞

D ∞ ∞ 2

And the minimum cost is an array of 4 integers in the order A, B, C, D:A advertises its minimum costs as: 0, 4, 1, ∞B advertises its minimum costs as: 4, 0, 3, 1D advertises its minimum costs as: ∞, 1, 2, 0

Show the first update on C’s table if C simultaneously receives packets with the minimum cost information from the routers A, B and D

The End.

Good luck!